Possible Mass Enhancement by Multipole Fluctuations Excited via the Singlet–Triplet Crystal Electric Field States in PrOs4Sb12: Sb-NMR Studies Using a Single Crystal

Abstract

121,123 Sb nuclear magnetic resonance (NMR) measurements of a high-quality single crystalline PrOs 4 Sb 12 have been carried out in magnetic fields up to 98 kOe and in the temperature range of 0.4–30 K. From the analysis of the 121 Sb Knight shift ( K ) and magnetic susceptibility at H ≈90 kOe, the transferred hyperfine coupling constants ( A THF ) are obtained. The Sb isotropic hyperfine coupling constant and anisotropic hyperfine coupling tensor are determined as ≈27.6 Oe/µ B and ≈(149.9,42.3,-192.2) Oe/µ B , respectively. The anisotropic part originates from the spin–dipolar field due to the local spin density at Sb 5 p orbital transferred through the hybridization with 4 f electrons. The results are consistent with the Sb 12 5 p molecular orbital with a u symmetry predicted from the band structure calculation. A THF for H ∥[001] is dependent on the magnetic field, suggesting the coupling of the Sb nuclear dipole moment with not only dipole moments but also octupole moments of the Pr 4 f electrons with the same time reversal. The NMR relaxation rate depends both on magnetic field and temperature, and the temperature dependence of 1/ T 1 is explained by the phenomenological expression 1/( T 1 T A THF 2 )∝(1/ T ) exp [-Δ NMR ( H )/ T ] with excitation gap Δ NMR ( H ). Here, we found that Δ NMR ( H ) is proportional to the largest excitation gap between Γ 1 and Γ t - of the triplet Γ 4 (2) state under the magnetic field. These results indicate that the crystal electric field (CEF) excitation and the degeneracy of the triplet state are important for understanding Sb-NMR results. It is strongly indicated that the mass enhancement in PrOs 4 Sb 12 is caused by the scattering conduction electrons with multipole fluctuations via the singlet–triplet CEF excitation.